Earlier reports from our laboratory demonstrated that hormones which activated adenylyl cyclase (catecholamines, glucagon, ACTH) in adipose cells inhibited insulin-stimulated glucose transport activity. Conversely hormones that inhibited adenylyl cyclase (adenosine, PGE1, nicotinic acid) activated transport activity. In contrast to insulin, these hormones did not alter the subcellular localization of either GLUT1 or GLUT4, but instead appeared to modulate transport activity at the level of plasma membrane. Recently we have investigated whether these changes in transport activity could be attributed to changes in the phosphorylation state of GLUT4. A partial phosphorylation of GLUT4 (0.1-0.2 mol/mol) observed in basal cells was unaltered by insulin but was slightly increased by isoproterenol. However this increased phosphorylation was confined to those transporters residing in the intracellular membranes. Furthermore it was observed both in the presence and absence of adenosine receptor activation whereas transport inhibition was only detected in the absence of adenosine agonists, suggesting that transporter phosphorylation does not explain the changes in transport activity. An alternative approach to answer these questions has involved the use of an impermeant [3H]-bis-mannose photolabel to monitor hormonally induced changes in accessibility of the glucose transporters in intact adipose cells. The inhibition of glucose transport activity induced by isoproterenol correlated closely with the decrease in GLUT4 accessibility. Isoproterenol appeared to induce a change in the conformation of the GLUT4 transporter such that it cannot either bind photolabel or transport glucose. Adenosine prevented these effects and rendered the transporter more accessible.